---
title: "Preloading"
subject: "Foundation Engineering"
chapter: 10
key: "semester_5/foundation_engineering:preloading"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/preloading
site: BCT Engineering
---

# Preloading

Foundation Engineering · Chapter 10: Foundation Soil Improvements

- Asked in: 2079 Bhadra, 2 marks; 2076 Chaitra, 2 marks; 2081 Baisakh, 1 marks (asked 3 times)
- Video: [Soil Stabilization | C7-L13 | Ground improvement: PRE LOADING](https://www.youtube.com/watch?v=pLOGaMc_tD0) — المقررات المفتوحة - Open Courses
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/preloading
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-10

## Question

What is preloading? Explain preloading method for improvement of foundation soil.

## Short answer

Preloading places a temporary surcharge fill on soft soil to induce settlement before construction. Vertical drains are often installed to accelerate consolidation; once about 90% consolidation is reached, the surcharge is removed and the structure built.

## Answer

**Preloading** is a soil improvement method in which a temporary load (surcharge), usually an earth fill, is placed on soft compressible soil before construction, so that most of the settlement takes place before the structure is built.

#### Method
1. The site is cleared and instrumented (settlement plates, piezometers).
2. A fill equal to or greater than the future structure load (a **surcharge**, often 1.2 to 1.5 times) is placed over the area.
3. The load is left in place until the required consolidation (generally 90%) is reached. Settlement is monitored, and the surcharge is removed, after which the structure is built with small additional settlement.
4. In thick clay layers the drainage path is long, so **vertical drains** (sand drains or prefabricated band drains) are installed to shorten it; the time of consolidation is $t = \dfrac{T_vH^2}{c_v}$ and reduces with the square of the drainage path.

```text
   Surcharge fill
   /////////////////
  =================  sand blanket
   | | | | | | |     vertical drains
  ~ soft clay ~~~~
```

**Advantages:** simple, cheap, and increases the strength of the clay. **Limitation:** takes a long time, and needs space and a fill material.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is preloading?
   A. Loading the structure after it is built
   B. Placing a temporary surcharge on soft soil before construction so most settlement occurs beforehand
   C. Compacting soil with a vibrating probe
   D. Injecting grout into soil
   Answer: B. A temporary load, usually earth fill, causes settlement before the structure is built. (Key point: Definition of preloading)

2. Why are vertical drains installed in thick clay under preloading?
   A. To stop water from leaving the clay
   B. To make the clay more plastic
   C. To shorten the drainage path and speed consolidation
   D. To increase the surcharge weight
   Answer: C. Time t = TvH^2/cv depends on the square of the drainage path, so shortening it greatly reduces time. (Key point: Role of vertical drains)

3. How long is the surcharge kept, and what is done then?
   A. Kept until about 90% consolidation, then removed and the structure built
   B. Kept permanently under the structure
   C. Removed as soon as it is placed
   D. Kept for one day, then structure built on top of it
   Answer: A. Settlement is monitored until the required consolidation (generally 90%) is reached, then the surcharge is removed. (Key point: Surcharge and removal)

## Related in Foundation Engineering

- [Purposes of foundation soil improvement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-purposes) — Foundation Engineering, chapter 10
- [Methods of soil improvement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-methods-list) — Foundation Engineering, chapter 10, asked 6×
- [Soil improvement techniques; explain two](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-explain-two) — Foundation Engineering, chapter 10, asked 2×
- [Preloading and stone columns](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apreloading-stone-columns) — Foundation Engineering, chapter 10

## Related in other subjects

- [Foundation settlement, effects, black cotton soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asettlement-black-cotton-062ba) — Building Technology, chapter 2
- [Soil exploration, safe bearing capacity, improvement](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexploration-bc-065s) — Building Technology, chapter 2